Difference between Binding Energy and Adsorption Energy

[cks]

Hi, all,

I find that binding energy (Van der Waals) are different from Adsorption Energy determined let's say from desorption spectroscopy.

If approaching from 3/2kT = eV -> then at room temperature, it's approximately 25meV. So, for adsorption to occur at room temperature, the binding strength has to be 25meV. However, I find that when scientists point to adsorption energy, the energy scale is approximately 1 order of magnitude higher? like a few hundreds of meV.

 

[mfb]

At a binding energy of 25 meV, bonds would break all the time (on the timescale of picoseconds), and you would not have a solid.
In order to have a fixed structure, the binding energy of a material has to be a (large) multiple of the thermal energy, such that broken bonds are extremely rare.

 

[gadong]

The energetics of physisorption (van der Waals interactions) differ from those of chemisorption. What kind of adsorption systems are you interested in?

 

[cks]

Hi, all,

I have found the answer from one paper (Molecular Physics, Vol. 103, No. 6–8, 20 March–20 April 2005, 883–890)

The adsorption energy Ea at a specific coverage is the difference between the total energy of the adsorbed
system and those of the clean substrate and the gas phase molecule (labelled ‘mol’). This adsorption energy
Ea can be further separated into two contributions: the binding energy (BE) characterizing the interaction of an
isolated adsorbate molecule with the metal, and Einter-ad that accounts for intermolecular interactions between
adsorbate molecules. Einter-ad can be evaluated as the difference in total energy between an array of molecules
in the adsorption geometry (without the substrate) and the same number of isolated molecules, both calculated
in a large cubic supercell (of length 17A ˚ ). Subsequently, BE can be computed as the difference between Ea and Einter-ad.